The distribution of time is essential within modern telecommunications networks based on PDH and SDH technology. These networks distribute a master PRC (primary reference clock) throughout the network which is based on a hierarchy of synchronisation levels often referred to as Stratum 1 (the PRC), Stratum 2 being levels synchronised to a Stratum 1 source, Stratum 3 being synchronised to Stratum 2 clocks and so forth.
Management of synchronisation of the clocks is referred to as clock discipline, or disciplining the clock.
Conventionally telecommunications networks are accurately synchronised in frequency with derived clocks that have controlled phase jitter and wander characteristic such as that specified in ITU G.812. However, these networks may have quite large phase offsets and do not have, nor require, absolute time synchronisation.
Mobile networks like GSM, WCDMA and LTE (FDD) operate in a similar way. The base stations require very accurate frequency control which is usually obtained by disciplining the base station clock to a traceable PRC via the core network. Typical base station clock accuracy has to be better than 50 ppb (parts per billion) with phase jitter and wander managed within the boundaries set by ITU G.812, for example. In these networks there is no need for absolute time (phase) synchronisation.
In mobile cellular networks the terminal is synchronised via the radio channel to the serving base station. The propagation time across this radio link is measured and controlled coarsely to roughly microsecond levels—generally for the purposes of managing the radio connections so that all the devices in the network, terminals and base stations, transmit and receive at the correct times.
Other mobile networks including CDMA (IS-95), CDMA2000 and WiMAX (802.16e) do however require accurate time synchronisation since they need both frequency and phase control in order to manage terminal mobility across the network. Obtaining accurate time synchronisation (both phase and frequency) is a somewhat more complex task than only frequency synchronisation, because this requires the slave clock to be corrected for the propagation delay from master to slave. There are, however, a few techniques for obtaining full time synchronisation, the two most widely used being NTP (network time protocol) and GPS, with others like IEEE 1588 and synchronous Ethernet being developed.
An interesting emerging market is for femtocells. These are very small cellular base stations that are connected to the core network via an IP (internet protocol) connection. This is a very elegant and flexible architecture that will allow operators to easily and rapidly provide high quality broadband mobile services in areas that are hard to cover—such as indoors. However, it has introduced the problem of synchronisation, because a general purpose IP connection does not include a clock distribution mechanism such as that present in SDH and PDH networks. Therefore most femtocell developers have had to resort to NTP, GPS or other techniques.
NTP is the most widely used protocol for time synchronisation of computers connected via the internet. The computer exchanges data packets with a time server using the network time protocol. This allows the machine to characterise the round-trip packet delay and then over a long period of time to determine the relative drift rate and offset of the local clock. This allows accurate frequency control to be achieved, and with suitable local oscillators and algorithms it is even possible to achieve frequency control better than 100 ppb. NTP also achieves a good level of time synchronisation by having measured the round-trip delay between computer and time server. With good quality symmetrical networks accuracies in the order of milliseconds can be achieved. This, therefore makes NTP suitable for use in femtocells on GSM, WCDMA and LTE networks, but its performance is not good enough for femtocells on CDMA, CDMA2000 and WiMAX networks.
However, even though NTP is good enough to use in femtocells for WCDMA and LTE networks, they do require a high quality oscillator which is expensive. With cost pressure on femtocells there is a strong desire to find more economical ways of achieving synchronisation.
Another widely used technique for achieving synchronisation in cellular networks, particularly for CDMA, CDMA2000 and WiMAX networks, is GPS. Although GPS was conceived and designed as a positioning system it can also be used as a high quality time source. This is because the satellites contain very stable clocks that are managed and synchronised via the ground control systems. Therefore when a GPS receiver receives signals from four satellites it is able to compute its position and time offset (x, y, z, t). Since the satellites are managed in a time synchronous manner this allows the GPS receiver to determine an accurate absolute time (in relation to the PRC of the GPS network). Many GPS receivers provide a time transfer function that is usually implemented as a physical signal from the receiver generated once per second. The accuracy of this time signal depends on the quality of the GPS receiver and the quality of the signals being received from the satellites. A top quality professional GPS time transfer receiver can achieve time synchronisation accuracies of 20 ns or better when installed and operated correctly. Even low cost GPS receivers are capable of time synchronisation to 100 ns. This makes GPS suitable as a time synchronisation method for femtocells on all current cellular networks. However, adding a GPS receiver adds cost and also restricts the locations where the femtocell can be deployed because GPS operation indoors cannot be guaranteed.
Another common technique used for synchronising femtocells is sometimes referred to as “macro cell sniffing”. The femtocell receives and decodes the downlink signal from one or more neighbouring base stations on the cellular network, as though it were a mobile terminal. Since the base stations are accurately synchronised this allows the femtocell to derive its synchronisation from the base station. This approach is workable for GSM, WCDMA and LTE networks in which the femtocell is in the coverage area of at least one other network cell, but it does not provide absolute time (phase) transfer, and therefore is not good enough for CDMA, CDMA2000 and WiMAX networks unless the precise positions of both femtocell and serving base station are known. However, should 3 or more base stations be receivable by the femtocell it can use measurements from all of them to compute its position (x, y) and clock offset using time difference of arrival algorithms, provided that the positions and synchronisation status of the base stations are known by, or can be supplied to, the femtocell. In practice this technique is not very useful for CDMA, CDMA2000 or WiMAX networks in which 3 or more base stations are seldom simultaneously receivable, but is a good solution for GSM, WCDMA and LTE networks where the femtocell is installed into an existing network within the coverage area of at least one base station.
A number of other techniques could be used for femtocell synchronisation and time transfer to mobile devices: broadcast infrastructure such as TV and radio transmissions; eLoran, which is presently proposed as a backup to GPS for transportation; other satellite navigation systems such as GLONASS, Galileo or COMPASS.
With the emergence of femtocells and also the migration of telecommunications networks towards the all-IP architecture the issue of time synchronisation is becoming significant. Several new approaches are in development or have been proposed:                IEEE 1588 is proposed as an enhanced protocol for synchronisation across IP. It includes layer 2 timing capability and methods for propagating physical timing information across switches, routers and other boundary equipment.        ITU G.8261 is a standard emerging from the ITU for time transfer across synchronous Ethernet networks. The ITU is also working on implementations for general IP and MPLS networks.        
One of the corollaries of being able to accurately measure time offsets in wireless systems is that given the near constant propagation speed of radio signals in air it is possible to also compute the position of the receiver or transmitter.
One of the best known such systems today is GPS. GPS works by measuring the time of arrival of signals transmitted by a constellation of 24 to 30 satellites orbiting the earth. At any one time up to 9 or 10 satellites may be in view. The receiver receives, decodes and measures the signals from the satellites and solves for the position (x, y, z) and time (t) offset of the receiver. Since the satellites are equipped with very accurate clocks that are synchronised precisely, the time offset represents the unknown offset of the receiver clock relative to the satellite system clock.
However, systems that use time measurement to compute the position of a receiver have been around for a long time and include well known systems such as DECCA and LORAN (now being modernised as eLORAN). The techniques used in many such systems can be traced back to the principles of U.S. Pat. No. 2,148,267. More recently systems that use measurements of cellular base stations, TV and radio broadcasts, or signals transmitted by other radio infrastructure such as WiFi Access Points have been developed. Some of these systems use Received Signal Strength (RSS) or Angle of Arrival (AOA) to determine the position of the receiver or transmitter, but these techniques are relatively imprecise and therefore high accuracy X,Y,Z positioning systems generally work using a few variants of time or range measurement:                TOA, Time Of Arrival—the system is operated using synchronised clocks and therefore the measurement can be equated directly to a range, or in the case of the receiver clock being unsynchronised as an apparent range comprising a measured range plus a range error introduced by the clock offset, as for example in GPS.        TDOA, Time Difference of Arrival—the transmitters are unsynchronised so the receiver uses the time difference between a pair of transmitters to determine a hyperbolic locus of possible positions. By using two or three pairs of the transmitters the intersections of these loci determines the position of the receiver. This technique is sometimes referred to as OTDOA (Observed TDOA) and in the case of GSM networks as E-OTD (Enhanced Observed Time Difference). Proponents of this approach include Cambridge Positioning Systems, for example U.S. Pat. No. 5,045,861 and Ericsson.        Round-trip time-of-flight—the mobile receiver retransmits signals received back to the transmitter thereby allowing a direct measurement of time-of-flight and thus range or double range. For example the system described in U.S. Pat. No. 6,611,234. Such systems may use a measurement of the reflected signal as used in radar systems, or they may actively retransmit the received signal—in which case knowledge of the “transponding” delay in the receiver is critical for accurate range measurement.        
In systems in which the transmitter clocks are unsynchronised a means to determine their relative offsets is required. This is usually done using one or more LMUs (Local Measurement Units). As for example is described in U.S. Pat. No. 7,260,407. By placing the LMU at a known location and measuring the time of arrival of signals from the unsynchronised transmitters it is possible to compute the relative time offsets and thereby reference all the transmitters to the LMU clock, or to an arbitrary virtual clock.
All of the above systems require an infrastructure—transmitters—with the precise positions of the transmitters being known. The positions of any LMUs that are used also need to be known. Some systems make use of a pre-existing infrastructure such as the base stations of a mobile cellular telecommunications network. Others deploy their own infrastructure, for example GPS.
Other systems have used the same concept but exchanged the functions of the transmitters and receivers. In these systems a mobile transmitter transmits a signal which is received and measured by a number of fixed receivers. Using measurements from at least 4 receivers the unknown position (x, y, z) and time (t) of the mobile transmitter can be determined, thereby allowing the transmitter to be tracked. In these systems the positions of the receivers need to be known precisely, and if they are unsynchronised a means of determining their clock offsets, such as the use of at least one LTU (local transmitter unit), is required to enable the receiver clock offsets to be determined. Examples of systems that use this approach are Uplink TDOA (True Position), Ultrawideband Systems and many of the commercially available systems falling into the category of Real Time Location Systems (RTLS).
This application puts forward a new technique that enables time synchronisation to be achieved between two or more devices that may be connected using either a wireless or wired connection, and which also enables the ranges or distances separating the devices to be determined, and hence their relative positions to be determined. It is therefore a combined ranging and time synchronisation technique for use on both wired and wireless networks.
When there are clusters of more than two devices, such as in a wireless network, it allows all devices to be accurately time synchronised and for the relative positions of them to be accurately determined. With three devices relative position and velocity on a 2D plane may be determined. With four or more devices 3D position and velocity can be determined. The system brings five significant improvements over current systems:                1. No infrastructure of transmitters or receivers is required—the devices determine their positions, velocities and time relative to other similar devices without the need for fixed infrastructure.        2. The way the signals are constructed makes the system agnostic of frequency band. Therefore it is not restricted to a particular band such as 2.4 GHz or UWB, and the techniques can be used in radiolocation systems as well as for systems using optical or acoustic communications.        3. The signals transmitted between devices to measure the range are transmitted asynchronously and the need for knowing time delays between received and transmitted signals is much less stringent than competing systems—some 6 orders of magnitude less so.        4. The system is unsynchronised and nor is it necessary to maintain a timing model that pseudo-synchronises device clocks, generally obtained by using LMUs (local measurement units) for fixed transmitters in traditional systems.        5. The system is symmetrical, or commutative, in that devices may compute their own position velocity and time (relative to the others), or they may compute the position, velocity and time of other devices in the network. It is, therefore, a combined location and tracking system. It is a system that could be used for the very accurate relative positioning of mobile wireless devices as an extension to existing standards such as 802.11, 802.15, WiMAX, DVB-T, DAB, cellular or others, or it could be implemented in an entirely proprietary way as a means of obtaining very accurate real-time position and time information. As such it has many uses in the fields of WSN (wireless sensor networks) and RTLS (real time locating systems), typically used in applications like: healthcare, asset tracking, emergency services, sport and others.        